For example, JP2010-281809A discloses an air flow meter that includes a sensor chip and a support plate. The sensor chip includes a sensor device formed in a thin wall portion of a substrate. The support plate supports the sensor chip. The sensor chip is installed in a recess of the support plate and forms a gap between the sensor chip and a wall of the recess to conduct air therethrough.
Therefore, in a case where a small amount of an assembly error (e.g., a gap dimension error of the gap formed between the sensor chip and the wall of the recess, or a tilt angle error of the sensor chip) occurs between the sensor chip and the support plate, an air flow rate of the air, which flows along a back surface of the thin wall portion having the sensor device, is changed, or the smooth flow of the air is disrupted. As a result, an output of the air flow meter relative to an air flow rate substantially deviates from a corresponding target value.
A digital multi-point adjusting technique (e.g., eight point or sixteen point adjusting technique) is known to adjust the output of the air flow meter to the corresponding target value.
Next, an example of previously proposed multi-point adjusting technique, which includes the following steps (i) to (iv), will be described.
(i) An air flow meter (hereinafter also referred to as a subject air flow meter), which is unadjusted, is installed to an apparatus that can change the air flow rate with a high accuracy.
(ii) An air flow rate, which corresponds to a corresponding one of adjustment points, is actually measured with the subject air flow meter. This step is repeated for all of the adjustment points.
(iii) Each adjacent two of the actual output values, which are outputted at the corresponding air flow rates (the adjustment points), respectively, are connected by a straight line, and unadjusted output characteristic is obtained through a linear approximation technique.
(iv) Adjustment data is obtained, and the obtained adjustment data is stored in an internal memory of the air flow meter. The adjustment data is data, which is used to adjust the output of the air flow meter to the target output (the target output characteristic, which is also referred to as adjusted ideal characteristic).
Here, an adjusting device, which adjusts the output of the air flow meter with the adjustment data stored in the internal memory, is provided in a sensor circuit (a control circuit) of the air flow meter. Therefore, when the above steps (i) to (iv) are performed, the output of the air flow meter can be adjusted to the target output (the target output characteristic).
However, in the previously proposed technique, the actual output value is obtained for all of the air flow rates, which are set at the adjustment points, respectively. Specifically, in the case of performing the eight point adjustment, the output of the air flow meter is measured for each of eight air flow rates. Also, in the case of performing the sixteen point adjustment, the output of the air flow meter is measured for each of sixteen air flow rates.
Therefore, the adjustment of one air flow meter takes a relatively long time. This will result in deterioration in the productivity of the air flow meter.